1. Field of the Invention
The present invention relates to a chain tensioner that includes a tensioner body having a cylindrical plunger bore with an open end, a cylindrical plunger with a bottom, slidably inserted in the plunger bore, and biasing means that biases the plunger in a protruding direction of the plunger and is accommodated so as to freely expand and contract inside a high pressure chamber formed to extend over interiors of the plunger bore and the plunger, the chain tensioner maintaining appropriate tension of a chain.
2. Description of the Related Art
It has been common practice to use a chain tensioner for maintaining appropriate tension of a chain. For example, a timing system has been known, which has an endless timing chain such as a roller chain passing over respective sprockets of a crankshaft and a cam shaft inside an engine room, and which uses a chain tensioner to bias a pivoting chain guide having a guide shoe that slidably guides the timing chain.
In such a timing system, as shown in FIG. 13, an endless timing chain CH passes over a drive sprocket S1 attached to a crankshaft and a pair of driven sprockets S2 and S3 attached to cam shafts in an engine room, and a chain guide mechanism in which this timing chain CH is guided by a pivoting chain guide G1 and a fixed chain guide G2 is formed.
The fixed chain guide G2 is fixed in the engine room with two mounting shafts B1 and B2, while the pivoting chain guide G1 is attached such as to be pivotable around the mounting shaft B0 in the plane in which the timing chain CH runs in the engine room.
The chain tensioner 500 presses the pivoting chain guide G1 and thereby maintains the appropriate tension of the timing chain CH as well as reduces its vibration.
The known chain tensioner 500 used in such a chain guide mechanism includes, for example, as shown schematically in FIG. 14, a tensioner body 510 having a cylindrical plunger bore 511 with an open end, a cylindrical plunger 520 with a bottom inserted in the plunger bore 511 to slide against the cylindrical surface 513 of the plunger bore 511, and biasing means that biases the plunger 520 in a protruding direction of the plunger 520 from the plunger bore 511.
The biasing means are formed by a coil spring 540 accommodated inside a cylindrical recess 521 in the cylindrical plunger 520 with a bottom and compressed between the plunger and the bottom 512 of the plunger bore 511.
Oil is supplied from an oil supply hole 514 formed in the plunger bore 511, so that a high pressure chamber 501 formed to extend over interiors of the plunger bore 511 and the cylindrical recess 521 of the plunger 520 is filled with oil. A check valve 550 (schematically shown as a check ball) stops the oil from flowing out from the oil supply hole 514.
As the plunger 520 reciprocates, oil flows through the small gap between the plunger 520 and the plunger bore 511, and the flow resistance of the oil provides the damping effect of slowing down the reciprocal movement of the plunger 520.
In such a timing system, how much reaction force is required of the coil spring 540 depends on the engine. If it is assumed that there is much looseness in the chain when the engine is started, a high reaction force is required.
A high reaction force is needed also when the chain has elongated due to aging. This reaction force is too high for the chain that is not elongated yet.
The tensioner 500 is therefore designed to press the chain with a force more than necessary, which would deteriorate friction properties between the chain and its running surface, and reduce fuel economy of the engine.
Also, the chain is subjected to tension fluctuations that occur in accordance with the engine rpm and changes in the load. Since the cycle and amplitude of tension fluctuations depend on driving conditions and are not uniform, the plunger reciprocates in various cycles and at various speeds to absorb the tension fluctuations.
The reaction force of the coil spring 540 is determined by the position of the plunger 520 in accordance with its constant spring constant, and the damping effect it provides is determined by the speed of the plunger 520. Therefore, it is difficult to always provide a suitable reaction force and damping force in response to various tension fluctuations, and there were possibilities that, in specific driving conditions, resonance might occur, or the tension fluctuations might be amplified.
To mitigate these problems, one known chain tensioner includes a nose portion provided at the tip of the plunger via a second spring (50) in addition to the spring (first spring 40) inside a high pressure chamber (fluid chamber 22) so as to absorb sudden tension fluctuations with this second spring (see Japanese Patent Application Laid-open No. H11-22792, etc).
Another known chain tensioner includes a center member in a middle part of a spring inside a high pressure chamber (fluid chamber 80). The center member reciprocates with expansion and contraction of the spring and has a predetermined mass. The center member has the same resonance frequency as that of the plunger (piston 40) so as to dampen the movement of the plunger (piston 40) (see Japanese Patent Application Laid-open No. 2000-145903, etc).